Expandable Polystyrene (EPS) has been made using suspension polymerization for over 50 years. Multiple formulation systems have been used depending on the equipment available, polymerization process used, and desired end use properties. EPS can be made using multiple techniques, including extrusion of polystyrene with addition of blowing agents through underwater dies to form expandable beads or suspension polymerization of styrene monomer with subsequent addition of blowing agents.
The addition of graphite and/or carbon black to make gray (or grey) EPS has been practiced for over 15 years. Grades and types of graphite and/or carbon black are selected to provide the desired end use properties. More specifically, carbon black and/or graphite can be added to EPS to improve the aesthetics or to improve the function of the finished EPS part. For improved aesthetics, generally the desired effect is to provide a consistent color. The amount of color can be measured visually by comparing to standards, or measured by instruments to determine reflectance, and color shifts.
For improved function, tests are normally run on a finished part. One example of improved function is for insulation grade EPS. Normal “white” grade EPS provides efficient, light weight insulation, and is widely used for home and commercial building construction. Improved insulation values have been obtained by adding graphite or carbon black to insulation grade EPS to give gray EPS. These grades provide improved insulation performance (measured as thermal conductivity, or lambda) vs. the normal “white” grades when formed at the same mass, or can provide equal insulation when formed at lesser mass.
For good color development in gray EPS, the carbon black and/or graphite must be dispersed efficiently within the EPS. This can be accomplished through the use of mechanical shear, dispersing agents, or a combination of dispersing agents with mechanical shear. Additionally, processes for production of carbon black and/or graphite can be modified to change the surface characteristics of the carbon black and/or graphite particles, and improve the ability to disperse these particles within polymers such as EPS.
The efficient mixing of carbon black and/or graphite in EPS can be made easier through the use of carbon black and/or graphite masterbatches, where relatively high concentrations of carbon black and/or graphite have been mixed with polystyrene under high shear conditions and then extruded to form pellets. The use of dispersing agents may also be used in combination with the high shear conditions to make masterbatches. The masterbatches also have the advantage of creating less dust while handling carbon black and/or graphite.
As previously noted, the addition of carbon black and/or graphite is known to improve the insulation values of EPS beads. For EPS produced using the extrusion method, the addition of carbon black and/or graphite is established art. However, producing gray EPS using the suspension polymerization method is more challenging. The addition of carbon black and/or graphite to the suspension formulation can create emulsion instability, leading to poor bead size distribution or even invert the emulsion from styrene in water to water in styrene, which can lead to gelling of the reactor. Previous art teaches that the suspension method may be used, but notes the addition of about 3-20% polystyrene to the styrene monomer improves the stability. It has been found that achieving desired bead distributions and cell structure can be difficult even using these levels or even higher amounts of polystyrene.
Additionally, the carbon black and/or graphite can interact with the free radical initiators, requiring much higher levels of the free radical initiators to achieve the targeted amount of polymerization. The interactions between the free radical initiators and carbon black and/or graphite can also create a variation in the polymerization rate.
Currently, preventing the instability requires careful monitoring of the reactor mass, including measuring the amount of conversion and estimation of bead size and distribution. Frequently, increases in the amount of radical initiator, surfactant and stabilizer are required to attempt to keep the emulsion stable with the proper bead size and bead size distribution. While this is possible in small laboratory and pilot reactors, it is much less practical for larger scale reactors.
Many EPS formulations already employ polyethylene wax as a processing agent to improve the cell structure of the EPS bead. Polyethylene waxes can also provide improvement for dispersion of carbon black or graphite within the EPS bead, as disclosed in W. P. Cottom, “Additives for Expanded Polystyrene Suspension Formulations”, IP.com, IPCOM000244928D, 1 Feb. 2016), as well as minimize the interaction between the carbon black and/or graphite and the free radical initiator. Other products disclosed included alcohols, ethoxylates, carboxylic acids, polymers, copolymers, oxidized polyethylenes, and modified waxes.
It has been confirmed that many of those products could minimize the interaction between the carbon black and/or graphite and the radical initiator. Additionally, adding up to 20% polystyrene can also improve stability. However, it was still very difficult to obtain the desired bead size and distribution using existing methods. The process required monitoring the conversion on a frequent basis, with multiple additions of radical initiator, surfactant, and dispersing agent even when making laboratory scale batches. Based on the inventors' experience, the process was not robust enough to transfer to commercial scale reactions.
It would thus be desirable to devise a method for preparing gray EPS with molecular weight, bead size, bead size distribution, and cell nucleation by suspension polymerization similar to those of white EPS on a consistent basis, but with a uniform color, or even darker color than is possible with known methods.